Vitamin B-12, or Cobalamin, is the largest and most complex vitamin. A slight deficiency of vitamin B-12 can lead to anemia, fatigue, mania, and depression, while a long term deficiency can cause permanent damage to the brain and central nervous system.It can only be manufactured by bacteria and only be found naturally in animal products, however, synthetic forms are widely available and many foods like cereals are fortified with Vitamin B12.